Acute kidney injury (AKI), an abrupt decline in kidney function, is a common and important problem among hospitalized patients. AKI is associated with significant short and long-term morbidity and mortality. Recent data suggest that the incidence of hospitalized AKI is on the rise nationally. Care for AKI is entirely supportive as there are no effective treatments currently. Thus, identifying opportunities to prevent AKI from developing in the first place is critically important. We have compelling preliminary data from Kaiser Permanente Northern California (a large integrated health care delivery system caring for ~3.3 million people) showing a reversal of the rising incidence of hospitalized AKI so that the incidence is now declining. In the proposed project, we will test the hypotheses that the declining AKI incidence correlates with temporal changes in medical practice such as reduced use of nephrotoxins (Aim 1). We will test the hypothesis that more cases of AKI identified in acute- care settings (e.g., emergency department and urgent care clinic) are being managed in the community setting; we describe these cases as community-acquired, community managed AKI (Aim 2). Finally, we will determine medical practices and patient characteristics associated with recurrent AKI, which contributes to both higher disease incidence as well as cumulative detrimental effects (Aim 3). Our research team has substantial expertise developing and analyzing datasets that comprehensively capture details of clinical care delivered and precise clinical outcomes among Kaiser Permanente members. The large scale of these datasets, along with the granular, linked individual-level hospitalization and ambulatory data available to us from the Kaiser Permanente electronic medical record, are particular strengths of our proposed approach. The findings generated by this research will not only fill important knowledge gaps in the contemporary epidemiology of AKI (including community-acquired, community-managed AKI and recurrent AKI) but will identify opportunities to reduce the risk of developing AKI. Since care of AKI is entirely supportive at present, if the drop in the incidence of AKI is associated wih specific changes in medical practice patterns, these can be prioritized for testing and implementation on a widespread basis across the United States. A nationwide reduction in the incidence of AKI would have direct and important benefits to both patients and the health care system.